Add new fields to Checkout that will appear on Inv

Currently in our shop only our salesrep enters the orders. The invoice is then forwarded to the customer. So that our sales rep does not have to create a separate account (with separate email addresses) for each customer I would like to add a few data fields into the Checkout module for him so that he can enter the customer name and other pertinent information that can then be added to the invoice. The thought is that the invoice will be emailed to the sales rep. He can then forward it to the customer.

Is it possible to add data fields to the checkout?